Start from the simpliest -replace your o2 sensor. This is what I

1) check your exhaust pipe, see if it is leaking smoke in the morning when you first start the car--Camries have very low pipes and they rust really fast. If you see a leak, get that fixed.if not

2) replace the easiest o2 sensor-- the one under hood on the exhaust manifold--a pipe. You can borrow a wrench and replace it yourself. O2 sensor costs $50 -$100. Have the code erased, drive it for a week.( I fixed my this way). If the check engine light comes back, replace the other one under the car...and repeat.

The o2 sensors are supposed to be replaced after certain years. They go bad...